This invention relates to apparatus for providing optical signals and more particularly to a highly efficient, low loss package and assembly for coupling optical signals from a pair of semiconductor laser diodes to an optical fiber.
Fiber optic communication systems such as those using graded index fiber have the potential for providing large system bandwidth and high information carrying capacity. Increased capacity would be very desirable for many types of communication systems. A significant example is data communications where a need exists for rapidly and reliably transmitting large volumes of digital information.
The system bandwidth and thus the information carrying capability of present fiber optic systems is limited by the optical sources presently is use. Signal sources using light emitting diodes optically coupled to optical fibers have the advantage of relatively low cost, but exhibit poor coupling efficiency in the range of about three to five percent with bandwidth limited to about two hundred megabits per second. Due to this undesirably low coupling efficiency, telecommunication grade laser sources have also been used. However, such laser sources having relatively long wavelengths in the 1300 or 1500 nanometer region, although otherwise generally satisfactory, are very costly and prohibitively expensive for many possible aplications.
As a result of these difficulties with light emitting diodes and long wavelength laser sources, it would be desirable to provide an optical source using readily available and relatively inexpensive short wavelength laser diodes. Such devices can be directly modulated at rates in excess of 500 megabits per second, thus offering the advantage of additional bandwidth and information carrying capability in a fiber optic communication system. A single laser diode can be optically coupled to an optical fiber with a coupling efficiency exceeding fifty percent. Despite such significant potential advantages, the problem of providing an acceptable semiconductor laser diode optical signal source has not been solved. One roadblock has been the inconsistent failure rate and questioned reliability of semiconductor laser diode devices. Another difficulty has been the failure prior to the present invention to provide a practical signal source package taking full advantage of the high potential coupling efficiency of a semiconductor laser diode in a rugged, reliable and inexpensive package.
United Kingdom patent application GB 2 098 353 A discloses an optical coupler for a redundant pair of semiconductor lasers, one serving as an operating light source and the other as a standby light source. The coupler includes a pair of polarization maintaining single mode fibers serving as confined light paths extending to adjacent faces of a polarizing beam splitter having its output connected to a main optical fiber. Although the use of redundant laser diode sources responds to the concern about long term unreliability of such devices, the disclosed optical coupler arrangement does not provide a practical signal source package and is subject to undesirably large coupling losses at both the semiconductor laser end and the polarized beam splitter end of each expensive and difficult to manufacture polarization maintaining single mode fiber.
U.S. Pat. No. 3,512,868 discloses apparatus for forming a beam of light in which light from a pair or some other number of light sources is polarized, reflected, transmitted and combined in order to provide a controlled beam of unpolarized light. Due to the use of techniques such as reflection and recombination of various light components, the arrangements disclosed in this patent are unsuitable for fiber optic communication systems. Moreover, the use of components such as quarter wave light retarders renders the apparatus undesirably frequency sensitive.
U.S. Pat. No. 4,588,262 discloses a multi-laser switch using redundant lasers coupled to an unspecified output by means of polarized beam splitters. Although this patent expressly recognizes the importance of eliminating all moving parts, nevertheless the switches there disclosed employ movable half wave plates rendering the switch expensive, sensitive and unsuitable for high reliability fiber optic communications systems. In addition, the use of half wave plates renders the switch undesirably frequency dependent.